Can anyone tell me if they've ever used a Check-Mate 25rd Mag in their M&P10, and if so did it perform properly? I wouldn't mind a little more firepower but I'm hesitant on buying a few without knowing if they fit & function properly.
CMI has the current military contract for M14 magazines and I have use the stainless version of their 20rd milspec mags in my M1A and they are great.
That said, the AR platform and the M14 platform don't use the same mag design. Some M-14 mags will barely fit in the M&P10 and some will be slightly too long and bind. You could probably get them to fit by grinding off the M14 mag catch which is on the back side and then machining an AR-type mag catch notch on the side of the mag body but I doubt the last round hold open would work without modifying the follower. Getting a modified mag to work on both platforms would probably require squishing the magazine lengthways slightly or milling a small notch in the M&P to clear the M14 catch (it would be harmless and not interfere with AR10 mags).
I know CA folks have gone to great lengths with getting pre-ban M14 mags to fit but I don't think it is worth it if you are not unfortunate enough to live there. Even that is obsolete because Armalite sells a replacement mag body to convert M-14 mags to AR10 mags that gets around the letter of the law.
It is something I considered and dismissed for myself (and I love projects).
